A brake disc of a disc braking system of a vehicle comprises an annular structure, or braking band, and a central attachment element, known as bell, by which the disc is attached to the rotating part of a suspension of a vehicle, such as a hub. The braking band is provided with opposite braking surfaces suitable for cooperating with friction elements (brake pads), housed in at least one caliper body arranged astride of such a braking band and integral with a non-rotating component of the vehicle suspension. The controlled interaction between the opposite brake pads and the opposed braking surfaces of the braking band by friction determine a braking action which allows the deceleration or stop of the vehicle.
In general, the brake disc is made of grey cast iron. This material allows, in fact, good braking performance to be obtained (especially in terms of containment of wear) at relatively low cost. Discs made of carbon or carbon-ceramic materials offer much better performance, but at much higher costs.
A limit not yet overcome of cast iron discs is related to the excessive weight of the disc.
Attempts have been made at overcoming this problem by making aluminium discs, with protective coatings. The protective coating is used on the one hand to reduce the disc wear and thus ensure performances similar to the cast iron discs, and on the other hand to protect the aluminium base from temperatures that are generated in braking, well above the softening temperatures of aluminium (200-400° C.).
Protective coatings currently available and applied on aluminium discs, while offering resistance to wear, however, are subject to flaking that cause the detachment thereof from the aluminium disc itself.
A protective coating of this type is described for example in U.S. Pat. No. 4,715,486, related to a low wear disc brake. The disc, made in particular of cast iron, has a coating made of a material in particle form deposited on the disc with a high kinetic energy impact technique. According to a first embodiment, the coating contains from 20% to 30% of tungsten carbide, 5% of nickel and the remaining part of a mixture of chromium carbide and tungsten. According to a second embodiment, the coating contains from 80% to 90% of tungsten carbide, up to 10% of cobalt, up to 5% of chromium and up to 5% carbon.
It has been found that one of the main causes of the detachment of traditional protective coatings from aluminium or aluminium alloy discs is the presence of nickel in the protective coating.
In case of application of the coating with flame spray techniques, a second cause of the detachment of traditional protective coatings from aluminium or aluminium alloy discs is the presence of free carbon in the protective coating. The carbon tends, in fact, to burn, combining with the oxygen incorporated in the protective coating being formed. This leads to the formation of micro-bubbles within the coating, which may prevent an adequate adhesion of the coating on the disc, favouring the detachment thereof.
From the foregoing it is clear that the coated discs made of aluminium or aluminium alloy cannot be currently used in the field of braking systems.
In consideration of the advantages arising from the use of aluminium to reduce the weight of brake discs, however, the need to solve the problems mentioned with reference to the prior art is very much felt in the field. In particular, the need of having aluminium discs with protective coatings that are able to increase the wear resistance of the disc and are at the same time resistant is felt.
One of the main problems of disc brakes is to extend the life of the braking surfaces and of the brake pads.
As known, in fact, in the braking surfaces, tracks or other surface irregularities are quickly created, for example due to the dirt or to the same friction material of the pads pulverised during the braking, which interpose between the braking surface and the pad during the braking action. These surface irregularities cause an irritating noise or rattle and a considerable increase of the wear of the disc and of the pad itself. In the practice, these drawbacks limit the braking disc life both for an inadequate service comfort and for excessive component wear.
The above wear problems are particularly accentuated in discs made with traditional materials, such as grey cast iron and aluminium or aluminium alloys, compared with discs made of carbon-ceramic materials.
The need to reduce the wear of the brake disc, in particular if made of aluminium or grey cast iron, and of the pads, therefore is particularly felt, so as to mitigate the above drawbacks.
On the brake disc side, the problem has been addressed by proposing brake discs coated with protective materials, designed to improve the wear resistance. In particular, a type of protective coating involves the use of a mixture of ceramic powders and binding metal materials.
On the brake pad side, the problem has been addressed in many ways, by proposing and testing different types of friction materials.
One type of friction material which is very widespread consists of materials obtained by hot moulding composed of fibres, abrasive materials, lubricants, metal materials and polymerizable binders (for example, phenolic resin).
An example of friction material is described in US20060151268. The friction material includes iron, aluminium, zinc and tin fibres. Iron fibres are present with an amount by volume of between about 1% and about 10%. Aluminium, zinc and tin fibres are present with an amount by volume of between about 1% and about 5%. The friction material is free from elemental copper, so as to prevent the release thereof to the environment during use. In particular, the friction material comprises about 11% by volume of graphite and/or coke, or about 6% by volume of graphite only. The material also comprises about 3% by volume of alumina, or about 5% by volume of zirconium oxide.
As is known, in order to obtain braking systems with good tribological performance and with high resistance to wear of the components, it is necessary to calibrate the brake disc features with those of the pad and vice versa. Often, in fact, components (discs and pads) that taken individually exhibit excellent performance (for example in terms of wear resistance), then give mediocre results, if not bad, when used in combination.
The need to identify disc brakes that provide good tribological performance and exalt the performance of the individual components subject to wear and tear (i.e. discs and pads) is therefore particularly felt.